Why does the ambient occlusion pass result in a black environment?
AO render pass result in a black environment cause it doesn't contain Alpha channel.To get any AO You have to have some objects in field of view, otherwise there is nothing to cause Ambient Occlusion.

There is a very simple way to achieve AO pass with transparent background.
In the node setup You have on the attached image (assuming, that You have transparent background enabled) You just need to attach Alpha output from Render Layer Node to Alpha input in Composite or Viewer node.